Thermal processors typically employ some type of mechanical transport system to move photothermographic film along a transport path through the processor during the development process. For various reasons, it is desirable to know the position of the film as it moves along the transport path, such as to determine whether a film jam has occurred in the transport system or to initiate various processor operations that are dependent on the film's position. As such, several systems have been developed to sense and monitor the position of film along the transport path.
One type of system employs mechanical switches which sense the position of film via physical contact with an actuator mechanism. However, the actuator mechanisms can scratch the sometimes delicate coatings on the photothermographic film or deposit contaminants on the film, both of which may result in defects or artifacts in the developed image. In efforts to avoid such drawbacks, non-contact methods of film detection have been developed.
One such method employs an ultrasonic sensor having a sound source located on one side of the film path and a sound receiver located on the other side, wherein a change in the sound level indicates the presence of film. While ultrasonic systems are effective at detecting film, the high cost of such systems has limited their application. Another method employs photoelectric sensors having a light source located on one side of the film path and a detector located on the other side, wherein a change in the detected light level from a fixed set-point indicates the presence of film. However, due to variables such as component variation, component aging, temperature drift, voltage drift, and light source drift, the amount of light provided by the light source and the light sensed by the detector can vary significantly over time. Consequently, in order to reliably detect film, it is necessary for such systems to employ a wide safety margin from the fixed set-point.
As a result, while such photoelectric sensors work well when the difference in light transmission across the film path is great between film and no-film conditions, they are not always effective at detecting film which is nearly transparent and does not provide a sufficient difference in the amount of light detected between film and no-film readings to enable reliable detection. To improve reliability, many photoelectric systems employ complicated circuitry to maintain the light output of the light source at near constant levels. However, such circuitry can be expensive and increases the space requirements of such sensors.
It is evident that there is a need for improved sensing of film movement through processors, particularly near-transparent film.